The transmission of optical signals in optical networks at bit rates in excess of 1 GHz, particularly over long distances, is susceptible to a number of sources of degradation to the optical signal. The present invention is concerned with correcting such degradation when it can be effected by applying equalisation to modify the amplitude waveform of an optical pulse and also to improvements in modulation for the purpose of regeneration.
It is known to provide detection of the waveform of an optical signal such as a pulse amplitude modulated optical signal and to subsequently apply equalisation to the resulting electrical waveform in order to cancel the effects of distortion by, for example, restoring symmetry. Such equalisation however becomes increasingly difficult with increasing bit rates and has the disadvantage that noise is added during the conversion to electrical signal with the result that the equalisation process may magnify such added noise. It would therefore be desirable to be able to effect equalisation in the optical domain. It is however difficult to implement such equalisation by known methods, a particular difficulty being that known techniques rely upon matching the operating wavelength of an optical filter to the carrier frequency of the optical signal. Fluctuation in the wavelength of the optical signal requires wavelength tracking or wavelength locking for stability. An etalon may, for example, be utilised to provide a narrow optical filter in such a process.
It is also known to provide a transversal filter in the optical domain comprising a branch, differential delay and recombining element. A further difficulty is that the optical signal generally has a finite line width which may cause multipath interference effects which degrade the optical signal during the filtering process. It has hitherto however not been possible to effect transversal filters in which negative weights are applied, ie. in which components of the optical signal are subtractively combined.
It is known from Idler et al (IEEE Photonics Technology Letters, Vol. 8, No. 9, Sep. 1996-"10 Gb/s Wavelength Conversion with Integrated Multiquantum-Well-Based 3-Port Mach-Zehnder Interferometer") to provide inversion of a single optical signal in addition to wavelength conversion by means of a Mach-Zehnder interferometer in which semiconductor optical amplifiers are utilised to set an interference condition between optical components of an input signal transmitted through first and second arms of the interferometer. A continuous wave optical signal propagated equally through the first and second arms is recombined to form an output signal which is modulated according to the interference condition and a pulsed optical signal is counterpropagated through only one of the arms so as to modulate the phase of one of the component signals by cross-phase modulation due to the non-linear characteristics of the semiconductor optical amplifier in that arm.